Low temperature susceptibility compensation

ABSTRACT

The NMR device with a magnet utilizes a composition exhibiting a desired value of magnetic susceptibility, wherein the composition comprises a metal ion selected from the group consisting of Gd +3 , Fe +3  and Mn +2  and an amorphous material using a ligand or chelating agent to solubilize the metal ion throughout the amorphous material, wherein the magnetic susceptibility of the composition exhibits a desired value at cryogenic temperatures such as nearly zero susceptibility at temperatures at or below 77 K.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a Divisional of U.S. patent application Ser.No. 10/669,772, filed Sep. 24, 2003 now U.S. Pat. No. 7,476,330.

BACKGROUND OF THE INVENTION

The invention is in the general area of magnetic susceptibilitycompensation and more specifically relates to the problem of reductionof magnetic inhomogeneities in nuclear magnetic resonance apparatus.

DISCUSSION OF RELATED ART

This invention relates to an amorphous composition, which has improvedlow temperature magnetic compensation, method of producing same, and usethereof. Although reference will be made to epoxy as an exemplarmaterial, this work is not so specifically limited.

A property of any material is its magnetic susceptibility, that is, thecoefficient relating an applied magnetic field to the induced magneticresponse of the material, as defined by the magnetization (inducedmagnetic moment or susceptibility)M=χH ₀ +S  Equ. 1Where H₀ is the magnetic environment or applied field, M is themagnetization and χ is the coefficient of magnetic susceptibility and Sis a constant for applied field values well above the saturation field,that is, H₀>>Hs. It is more precise to define the coefficient χ as∂M/∂H ₀=χthrough recognition that there is a significant temperature dependenceas well as non-linearities associated with the strength of the appliedfield. It should also be understood that the saturation field is aproperty of materials generally because magnetization induced by anexternal field does not increase without limit in response to anincreased value of said field. Rather, the magnetization M, or magneticmoment, saturates at an external field intensity characteristic of thematerial for a temperature T.

Values of χ>0 are said to be paramagnetic and values less than 0 arediamagnetic. The vector character of the quantities M and H₀ need not beconsidered for the purposes of this work. It is readily appreciated thata non-zero value for χ means that in proximity to the material, theactual field will include the applied field and a contribution from themagnetic moment (M) of the material. This local effect can contributeinhomogeneities (gradients) to the field and therefore disturb magneticresonance measurements for which the magnetic environment must beextremely uniform and any gradients are precisely controlled in timedirection and magnitude. In what follows, the term “susceptibility” willbe understood in the context to refer to the coefficient ofsusceptibility χ, or the physical effect, M, as the context mayindicate. This distinction is critical to understanding the distinctionof the invention over the prior art. It is clear that in a magneticfield H₀, where the value of M for a given structure is equal to thevalue of M for the surrounding medium, there is no magnetic gradient andthe structure is indistinguishable (magnetically). At ordinarytemperatures and field magnitude well above saturation, equation 1 is asimple linear relationship for (non-ferromagnetic) materials. Bymatching the coefficient of susceptibility χ, for two materials, forexample a structure and the surrounding medium, the prior art achievedthe desired result over a range of values for the magnitude of the fieldH₀. The present invention matches the value of M for a given field H₀without regard to the coefficients of susceptibility for the respectivematerials. Although the generic term “susceptibility compensation” iscommonly employed, it is important to recognize this distinction

In the art, such as encountered in high resolution applications ofNuclear Magnetic Resonance (NMR), materials having a very low, or nearlyzero magnetic susceptibility are desired for structures proximate thesample space where magnetic inhomogeneities distort the magnetic fieldenvironment. Undesired magnetic susceptibility of materials formingstructures subject to a magnetic environment has been compensated usinga variety of methods, including use of different materials intimatelyrelated (as with coatings), combined metal mixtures, gaseouscombinations, and liquids, and generally as compensating agents. Itshould be recognized that these techniques often take the form ofreduction of magnetic susceptibility discontinuities between differentmaterials, or the achievement of a desired value for the magneticsusceptibility M, of some object.

In instances where the perturbations due to nonzero susceptibilitypersist, magnet shim coils are commonly used to provide a controlledincremental magnetic field to compensate for magnetic inhomogeneities.The successful practice of shim coil compensation requires theperturbations be weak and slowly varying near the active sample volumeof the NMR probe. For example, U.S. Pat. No. 3,091,732 is directed toeliminating undesired magnetic field gradients across a sample spacecaused by paramagnetic or diamagnetic discontinuities of members locatedin the vicinity of the sample space. This is accomplished by combiningselected proportions of paramagnetic and diamagnetic materials to matchthe magnetic susceptibilities.

Another example of prior art is U.S. Pat. No. 5,545,994, which disclosesadding a mixture of two fluids of different specific magneticsusceptibilities and at respective partial pressures to a sample spaceto achieve a desired volume magnetic susceptibility which matches thevolume magnetic susceptibility of the solid component.

A further example is U.S. Pat. No. 5,684,401 which describescompensation of magnetic susceptibility variation using a perfluorinatedhydrocarbon as a matching medium enveloping a microcoil to minimizemagnetic susceptibility induced variations in the Bo magnetic field.

In all of these examples, the practice required little or no knowledgeof the nonlinear properties of magnetism. This is a result of theapplication of the art to situations of small susceptibility andenvironments at or above room temperature.

In most practical situations structural members of a magneticallysensitive apparatus exhibit “invisibility” in the magnetic sense fromthe environment of such structural members. Accordingly, matching themagnetic properties of such structural members to their environmentcontributes to the magnetic homogeneity of the region occupied by thosestructural members and their environment. If the environment is vacuum,(or air), the magnetic susceptibility thereof is zero, (or nearly zero).It is therefore desired to so modify the structural member that itsmagnetic susceptibility is likewise zero. If the environment is notvacuum or air, but some other material exhibiting magneticsusceptibility M₀, it is desired to modify the structural member tosimilarly have susceptibility M₀. In this work, it should be understoodfrom the context that a null value for M is merely a particular valueselected for the above purpose.

In the fabrication of magnetic components, epoxy is often used as abonding agent for cementing substances because of its propertiesunrelated to magnetic susceptibility. The magnetic susceptibility ofepoxy in combination with its use in small, localized areas near the NMRsample space require that this material be compensated. Typical epoxiesin common commercial use are characterized by diamagnetic coefficient χin a range −1 to −0.5×10⁻⁶ cgs units. The usual practice forsusceptibility compensation is to follow the art mentioned above, andcombine or mix a paramagnetic material into the epoxy such that thesusceptibilities appreciably cancel. This practice becomes difficult atcryogenic temperatures due to the non-linear and poorly characterizedmagnetic behavior of many paramagnetic materials at low temperatures.

In the present work, reference to an epoxy is understood as aparticularly useful example of the invention and does not exclude otheramorphous materials from the scope of the invention.

Accordingly, in the art, there exists a need for a material orcomposition that exhibits a desired value of magnetic susceptibility. Itis particularly useful to achieve a nearly zero magnetic susceptibilityat low (“cryogenic”) temperatures. It is appropriate to identify anupper limit for a cryogenic temperature by the condition where thealignment energy of the molecular magnetic moment in an applied field H₀for the composition approaches a value comparable to the thermal energy,i.e.,μ_(g) H ₀ /kT≈1where μ_(g) is the elemental (molecular, or atomic) magnetic moment, kis the Boltzman constant and T is the absolute temperature.

The composition of the invention is intended for use at a cryogenictemperature as a structural material itself, or as a bonding orcementing substance. In this work, “low temperature” is identified withcryogenic temperatures generally. Practical applications commonly occurat or below about 77 K and this is often cited herein as an exemplarycryogenic temperature.

SUMMARY OF THE INVENTION

Magnetic resonance instrumentation functions in an environment of ahomogeneous magnetic field and/or precisely controlled field gradients.Field homogeneity is disturbed by any object within the fieldcharacterized by a value of magnetic susceptibility differing from thecorresponding value for the environment of the object. Therefore, thecontrol of the magnetic homogeneity of a material allows structures ofsuch material to be essentially indistinguishable by magnetic means. Inmost contexts the relevant environment is vacuum or air, e.g., magneticsusceptibility, M=0 and χ=0 (very nearly so for air). The presentinvention is directed to providing an amorphous material, such as anepoxy that is matched to a selected value (ordinarily a null) ofsusceptibility, and moreover, to maintain this value while subject to aselected magnetic field intensity at range of cryogenic temperatures,and in another embodiment, to achieve this value at a particularcryogenic temperature.

One object of the invention is to modify the magnetic susceptibilityproperty of an epoxy, or similar amorphous composition which is usableas a structural material itself and/or as a bonding material, and whichhas nearly zero magnetic moment at cryogenic temperatures and at themagnitude of the magnetic fields characteristic of the NMR apparatus.

A further object is to modify the magnetic susceptibility property ofepoxy or similar amorphous material by mixing a metal ion selected fromthe group consisting of gadolinium (Gd³⁺), tri-valent iron (Fe⁺³) anddi-valent manganese (Mn⁺²) together with a ligand or chelating agent,into the epoxy (or similar amorphous material), whereby to achieve adesired value for the magnetic susceptibility at a cryogenictemperature.

Another object is to provide an NMR apparatus which uses an epoxycomposition, such as for cementing magnetic materials, which compositionhas nearly zero magnetic moment at low temperatures.

A still further object is to provide an epoxy (or similar amorphous)composition comprising a metal ion selected from the group consisting ofGd³⁺, Fe⁺³, and Mn⁺² which is solubilized in the epoxy using a chelatingagent or ligand so that the magnetic susceptibility of the resultingcomposition has nearly zero magnetic moment at temperatures of 77 K andlower.

The foregoing and other objects are attained by the invention whichencompasses an epoxy (or similar amorphous) composition which includes ametal ion selected from the group consisting of Gd³⁺, Fe⁺³ and Mn⁺²dissolved in the epoxy using a chelating agent or ligand, wherein themagnetic susceptibility, or magnetic moment at desired applied fieldstrength, of the resulting epoxy composition is nearly zero at lowtemperatures of 77 K or less. Thus, the invention can be used to bond orcement together magnetic materials or be used as a structural materialitself, without requiring any further additional compensating materialto compensate for undesired magnetic susceptibility.

The inventive composition comprises a metal ion selected from the groupconsisting of Gd³⁺, Fe⁺³ and Mn⁺² solubilized in the epoxy (or similaramorphous) using a chelating agent or ligand, such as Lg, as describedbelow. The epoxy may be epoxy resin and its curing agent, and thecomposition includes other elements as described below. The metal ionsused in the invention exhibit advantageous magnetic susceptibilityproperties at low temperatures because of their nearly symmetricalground states for orbital angular momentum, i.e. L=0, and simplicity inelectrical configuration so as to fit the Brillouin-Langevin equationover a wide range of field strength and temperature.

Thus, use of the inventive composition is especially advantageous in thedesign formulation of magnetic materials that have nearly zero magneticsusceptibility at low temperatures. For example, magnetic materials usedin NMR apparatus are desired to have low or nearly zero magneticsusceptibility, especially at low temperatures, for greater sensitivity,reliability and reduced noise. The epoxy composition of the inventioncan be used, advantageously, to fabricate for example by bonding,magnetic material parts, with the additional advantage that theinvention epoxy composition does not add further magnetic susceptibilityat low temperatures. Thus, structural components for use in an NMRmagnetic field, fabricated using the inventive composition would havetotal magnetic moment of nearly zero at low temperatures. Alternatively,these parts are fabricated to exhibit a vanishing induced magneticmoment at a specific (relatively high) applied magnetic field. In thatmanner, adverse effects due to unwanted magnetic susceptibility orinduced magnetic moment would be further reduced.

The invention comprehends a composition as described herein forexhibiting desired susceptibility at cryogenic temperatures, the methodof preparing such composition and an NMR apparatus employing suchcomposition. The modified amorphous composition of the invention isaccomplished by tailoring the magnetic properties of a selectedamorphous material, principally an epoxy or a plastic or glass throughan appropriate admixture of a selected metal ion selected from the groupconsisting of Gd³⁺, Fe³⁺, and Mn⁺². The preparation of the compositionentails achieving solubility of the selected metal ion into the selectedamorphous matrix. This is accomplished by incorporation of the selectedion into a chelating agent or ligand that exhibits solubility with theamorphous matrix. The tailoring of the magnetic susceptibility propertyis directed to achieving a fixed value of the induced magnetization ofthe composition at cryogenic temperature in respect of another material,where that other material ordinarily comprises the environment for theamorphous composition of the invention. The composition is employed inproximity to the sensitive volume of an NMR apparatus. An especiallyuseful composition is an epoxy where the bonding properties of the epoxyare required for structural purposes.

One aspect of the invention is an amorphous composition for magneticsusceptibility compensation at cryogenic temperatures to which there isadded a metal ion selected from the group consisting of Gd⁺³, Fe⁺³ andMn⁺² and a ligand or chelating agent whereby the resulting compositionis characterized by having nearly zero magnetic susceptibility atcryogenic temperatures.

A second aspect of the invention is a method of preparing asusceptibility compensated amorphous composition for use at lowtemperatures, comprising the steps of mixing a metal ion selected fromthe group consisting of Gd³⁺, Fe⁺³ and Mn⁺² with such amorphous materialand a chelating agent or ligand so that the resulting composition has aselected magnetic susceptibility at low temperatures.

A third aspect of the invention is an NMR device utilizing a magneticmaterial fabricated to produce an amorphous composition to which therehas been added a metal ion selected from the group consisting of Gd³⁺,Fe⁺³ and Mn⁺², and a ligand or chelating agent and having the propertyof a selected value of magnetic susceptibility at cryogenictemperatures.

The invention is further described below for illustrative purposes andsuch description is not to be considered to be limiting in any way.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic graphical explanation for the process ofsusceptibility matching to a desired value.

FIG. 2 is a schematic graphical explanation of field specific magneticmoment nulling.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an NMR apparatus, for example, use is made of a magnet, and mostoften, a superconducting magnet having a bore. The magnet bore maysupport properties producing certain disadvantageous effects, such asmagnetic residual fields, electrical noise, inhomogeneity, etc, due tomagnetic susceptibility discontinuity of materials comprising, orsupported within the bore. By using selected materials having selectedvalues of magnetic susceptibility, the magnetic susceptibilitydiscontinuity effect can be reduced.

In fabricating the NMR apparatus, epoxy resin is often used as a bondingor cementing agent to join together components of the magnet and/or ofthe NMR probe supported within the bore. Even though used in smallquantities, epoxy resin has a non-zero value of magnetic susceptibility.

The invention encompasses an epoxy composition which has nearly zeromagnetic susceptibility and/or induced magnetic moment, especially atcryogenic temperatures and high applied field intensity, such as at orbelow 77 K and of the order of tens of tesla.

FIG. 1 shows the general methodology of susceptibility matching, that ismatching the value of the induced magnetic moment of one material toanother (the latter usually being the environment of the former). Thetwo substances, A and B, exhibit respective (induced) magnetic momentsas a function of applied field. An appropriate combination of thesubstances produces a resultant selected value of ∂M/∂H₀≈0 over a widerange of field beyond a low field region. It should be apparent that thevalue of M, the induced moment, may be independently selected. A valueof zero for M is appropriate to the case where the magnetic environmentof the subject material is zero. This is the usual situation where thebore of an NMR magnet is vacuum or air and the subject material isdisposed within that bore. (It should be recognized that an NMR magnetbore or field region generally may be enveloped by some other material,a fluid perhaps, exhibiting some non-zero applied moment M orsusceptibility coefficient χ. The present discussion is directed tomatching the induced moment to a selected value at cryogenictemperatures.

FIG. 2 represents a different application of material property tailoringof effective magnetic moment of a material to exhibit a lineardependence and to vanish at a particular value of applied magneticfield. A first material, symbolically M_(A) is saturated above somefield strength, here shown as about 5 Tesla. A second material, M_(B)exhibits a diamagnetic susceptibility above the same field value. Anappropriate admixture, M_(A)+M_(B) of the two materials will provide anull for the magnetization of the composite material, at a narrow fieldvalue. Where the constituent materials closely follow the B-L equation,that critical field value can be calculated and the composite can be sodesigned to yield this result. Note that the susceptibility χ is neithernulled nor matched to any other susceptibility coefficient for thisembodiment of the invention

Returning now to FIG. 1, under normal conditions, prior art epoxycompositions have some undesired magnetic properties at varioustemperatures including low temperatures. For the sake of example, it maybe noted that common epoxies in combination with their curing agentshave bulk coefficients of susceptibility characteristic of organiccompounds. That is, these materials, in general are diamagnetic with acoefficient of susceptibility of the order −1×10⁻⁶ cgs units. A null inthe magnetic susceptibility of the inventive epoxy composition can beproduced at temperatures at or below 77 K in the presence of asaturating magnetic field H₀ when a metal ion selected from the groupconsisting of Gd³⁺, Fe⁺³ and Mn⁺² is chelated or solubilized with epoxy,using a ligand or chelating agent to form the desired molecularstructure. As further discussed below, other amorphous matrices may beemployed in place of epoxy, where desired.

Other metal ions, e.g., holmium, have been investigated in the course ofthis work. When appropriately chelated and introduced with an epoxyresin, these have produced low magnetic susceptibility at highertemperatures where the temperature dependence of magnetic susceptibilityfollows Curie's Law. Thus, for cryogenic temperatures, such as below orat 77 K, these other metal ions have proven unsatisfactory for producingthe desired magnetic susceptibility properties. The epoxy compositioncould be used advantageously with those other selected ions which, whensubjected to such low temperatures produced the total effect of nearlyzero magnetic susceptibility, M, for the epoxy composition of theinvention. Thus, further compensation is not needed to obtain thedesired uniform magnetic environment within the bore of the NMRapparatus.

This novel discovery of the null in magnetic susceptibility of the epoxycomposition of the invention, as chelated with a metal ion selected fromthe group consisting of Gd³⁺, Fe⁺³ and Mn⁺² using a chelating agent orligand, enables use of such inventive composition to produce anunexpected and outstanding result. The operation of shimming themagnetic field, (modifying the prevailing magnetic field with locallyapplied field components) is substantially reduced where the presentinvention is employed because magnetic irregularities of the structurehave been substantially reduced or eliminated.

It is known that certain metal ions (Σ^(+n)) are paramagnetic, i.e.magnetic susceptibility χ is greater than zero, and can be mixed withepoxy resin produce a null in magnetic susceptibility. That is:χ_(Σ) V _(Σ)+χ_(R) V _(R)=0wherein X_(Σ) is the magnetic susceptibility of the metal ion, V_(Σ) isthe volume of the metal ion, X_(R) is the magnetic susceptibility of theepoxy resin, and V_(R) is the volume of the epoxy resin. This is theessence of the manner of susceptibility matching practiced in the priorart

Generally, the magnetic susceptibility coefficient of the metal iondepends on the temperature, i.e. χ_(Σ)(T). Thus, compensation isachieved only at one temperature for a given volume of the metal ionV_(Σ). This holds for a wide range of magnetic fields.

For most materials at or near room temperature subject to a magneticfield above the saturation point of such material, the magnetic momentper unit volume can be quantitatively expressed as M=χ(T)H₀+S, whereinH₀ is the applied magnetic field, χ (T) is the magnetic susceptibilitycoefficient which is a function of temperature T and S is a constant.

To obtain cancellation of induced magnetic moments of two materials, Aand B, these materials are selected so that M_(A)=V_(A)χ_(A)(T)H_(o)+S_(A) and M_(B)=V_(B) χ_(B)(T)H_(o)+S_(B), that isV_(A)M_(A)+V_(B)M_(B)=0, and the magnetic moments of the two materialscancel each other at a particular temperature T. The value of zero forthe combination of the two materials is selected to match a surroundingmedium such as vacuum or air, which has zero (or nearly zero) magneticmoment. A value for M other than zero, e.g., M′, would be appropriate toa surrounding medium for which M′ is the magnetization value under thesame conditions.

At ambient temperatures, S_(A) and S_(B) are both very small as comparedto χ_(A)(T)H_(o) and χ_(B)T)H_(o), and then V_(A) χ_(A)(T)=−V_(B)χ_(B)(T). In that case, as shown in FIG. 1, there is cancellation of themagnetic susceptibilities, which are independent of magnetic field.

At cryogenic temperatures, when S_(A) and S_(B) are large, or when themagnetic moments M_(A) and/or M_(B) is a non-linear function of magneticfield, the magnetic susceptibility M_(effective) of the combinationcannot be matched to the desired value for all field strengths. For aninstrument operating at a fixed field strength, this presents nopractical limitation.

At low temperatures, paramagnetic ions are non-linear in their magneticproperties and the saturation point is temperature dependent. At hightemperatures, paramagnetic ions have a susceptibility coefficient thatvaries inversely with temperature, that is χ(T)=C/T wherein C is aconstant. Thus, prior art susceptibility matching is easily adapted to arange of temperatures in those temperature regions where Curie's lawholds.

The situation is more complex for cryogenic temperatures, that is at 77K or less where the magnetic moment for atomic systems of simplesymmetry follow the Brillouin-Langevin equation, rather than a simplelinear dependence on H₀ through a simple susceptibility coefficient χ Atlow temperature and high fields the magnetic moment, M, of an ion willbe a non-linear function of magnetic field and temperature. In the artof susceptibility matching for applications to high resolution NMR, theactual operational magnetic field can be high, e.g., 20 Tesla. Thebehaviour of magnetic ions at this field strength is often impracticaland expensive to directly measure. However, it was found that forcertain classes of ions having a symmetric ground state orbital angularmomentum L=0, the magnetic behaviour closely follows theBrillouin-Langevin equation. Using this quantitative behaviour allowscharacterization at low magnetic fields with confident extrapolation tothe very high magnetic fields used during NMR measurements, at cryogenictemperatures, such as at or below 77 K.

The metal ions which meet the foregoing criteria were determined to begadolinium (Gd³⁺), iron (Fe⁺³), and manganese (Mn⁺²). These metal ionsof the invention are simple in electrical configuration and follow theLangevin behaviour so that at low temperatures magnetic behaviour ispredictable.

A discussion of the characteristics of the metal ions of Gd³⁺, Fe⁺³ andMn⁺² is disclosed, for example, in Ashcroft and Mermin, “Solid StatePhysics”, 1975, Rinehart, Holt and Winston, Tables 31.3 and 31.4, made apart hereof.

The metal ions of this invention, when integrated into, dissolved into,or mixed with other elements and/or compounds can readily be matched inmagnetic susceptibilities with air or vacuum. A preferred embodiment isthe case where the metal ion is selected from the group consisting ofGd³⁺, Fe⁺³ and Mn⁺² and is chelated or solubilized into epoxy resin orother material so that the resulting composition has nearly zeromagnetic moment, M. The epoxy resin when chelated with the metal ion ofthe invention can be used as a bonding material for other magneticmaterials so that the resulting structure is almost totally free ofundesired magnetic susceptibility. The inventive composition can be usedalso for structural purposes.

In the prior art described in U.S. Pat. No. 3,091,732, a metal ion andepoxy resin formed a mechanical mixture. For example, a metal oxide(MnO₂) was used in a finely powdered state and dispersed finely anduniformly in the epoxy resin. However, such mixture does not provide anearly zero magnetic susceptibility at low temperatures, and hence wasnot satisfactory.

In contrast, the present invention prescribes that a metal ion selectedfrom the group consisting of Gd³⁺, Fe⁺³ and Mn⁺² is chelated with acoordination donor ligand (called “Lg”) so as to render the metal ionsoluble in the epoxy. The ligand, Lg, or chelating agent, has sufficientpolar or non-polar characteristics to be soluble in the epoxy.

Gd(ACAC)_(3′), that is Gd(III) acetylacetonate, and Gd(mthd)₃, that is,Gadolinium tris 2,2,6,6-tetramethyl-3,5-heptanedionate, are twoexemplary compounds that have been successfully used in the invention.These compounds preferably include the Gd metal ion. Alternatively, theFe⁺³ and Mn⁺² metal ion are advantageous in a greater range of magneticfields and at temperatures of 77 K and lower, although their solubilityin epoxy resin may not be perfect.

For simple ions, the magnetic moment obeys the Brillouinn-Langevinequation (“B-L equation”) which is discussed in detail in the aboverecited Ashcroft and Mermin reference “Solid State Physics” and which ismade a part hereof by reference. This non-linear behaviour of themagnetic susceptibility of the above mentioned ions originate in thefixed magnitude of the atomic magnetic moment, and as well in thesaturability of the susceptibility. A balancing of magnetic moments fromthe ion and resin at all temperatures and magnetic fields is difficultin the prior art.

However, if the metal ion follows the B-L equation, formulations can beused in which the magnetic moments cancel at specified magnetic fieldsand temperatures. For example, for magnetic fields above a particularvalue at a given temperature, it can be arranged thatM _(ion)(saturated)+χ_(R) H ₀≈0where M is the magnetization (or magnetic moment per unit volume, orsimply “induced magnetic moment”) and H₀ is an applied external magneticfield and χ_(R) is the magnetic susceptibility of a selected epoxyresin.

In addition to the use of the inventive epoxy composition as a bondingor cementing material for fabrication of components in a magneticenvironment, the epoxy composition of the invention has otheradvantageous applications, such as for structural uses. For example, theinventive composition can be used to provide complete support of inserts(probe components), as supporting structures, as potting for structures,etc, as used in NMR apparatus. In such applications, there arelimitations resulting from dielectric loss and NMR signal backgroundlevels, characteristic of room temperature epoxy resin compositions. Atcryogenic temperatures, such parasitic limitations are reduced ineffect. Also, the NMR background noise is reduced because the NMRrelaxation time becomes long. Further, the dielectric loss is reduceddue to slowing of the electric dipole reorientation.

Moreover, the principles of the invention are applicable withcompositions other than epoxy resins, such as plastics, glass, resins ofother matrices, etc. An example is the addition of Gd₂O₃ in borosilicateglass. The usage of the inventive composition is advantageous wherecryogenic temperatures, such as 77K or lower, are used, since it is atthese lower temperatures that the inventive composition may be easilymanipulated to exhibit nearly zero magnetic susceptibility. Accordingly,the invention is applicable for bonding or cementing materials in amagnetic environment, as well as for the structures themselves.

The symbol “Lg” is used herein for any ligand or chelating agent, andall such terms may be used interchangeably herein and are to beunderstood as such. In one experiment using Gd for modifying an epoxy,2,2,6,6-tetramethyl-3,5-heptanedionate was selected for the solubilizingligand. A solubilizing ligand for use herein should more or less conformto the rules of solubility, namely, that likes dissolve likes, such aspolar solvents dissolve polar solutes, and non-polar solvents dissolvenon-polar solutes. Also, although it is preferred herein to use epoxyand/or epoxy resin, it is to be understood that the invention is notlimited thereto and can be readily extended to various amorphousmaterials such as resins of various types, and other materials, such asglass, plastics, etc. The above materials are limited to a class capableof maintaining the chelated modifying ion in solution at the lowtemperatures contemplated. The ligand is bound to the metal ion and isconnected to the host material. In a sense, the ligand surrounds themetal ion and is solubilized in the host material, such as the exampleof the epoxy. It is to be understood that any ligand or chelating agentor solubilizing agent that binds the metal ion and effects solubility inthe host material, such as the epoxy resin, is within the scope of theinvention. That is, the ligand binds the metal ion, which forms acoordination complex with the host material. Another way to considerthis invention is as a chelating process wherein the metal ion isattached to neighboring host material atoms by at least two coordinationbonds in such a manner as to form a closed chain.

The foregoing is descriptive of the invention. Numerous extensions andmodifications thereof would be apparent to the worker skilled in theart. All such extensions and modifications are to be considered to bewithin the spirit and scope of the invention.

1. An NMR apparatus comprising a magnet for producing a polarizing fieldand utilizing a composition subject to said polarizing field, saidcomposition an amorphous comprising a selected amorphous material and ametal ion selected from the group consisting of Gd³⁺, Fe⁺³ and Mn⁺², anda ligand said composition having a selected value of magnetization atcryogenic temperatures.
 2. The NMR apparatus of claim 1, wherein saidligand binds said metal ion and effects solubility thereof in saidcomposition.
 3. The NMR apparatus of claim 1, wherein said cryogenictemperatures are at or below 77 K.
 4. The NMR apparatus of claim 1,wherein said composition is surrounded by a material exhibiting amagnetization of zero and said selected value is zero.
 5. The NMRapparatus of claim 1, wherein the selected amorphous material comprisesa glass or plastic.